A system which operates with fluids under pressure commonly requires a pressure regulator to assure that fluid from a source under high pressure is supplied to the system at low, substantially steady pressure. The pressure regulator must maintain a steady pressure even though variations in the fluid pressure at the source may occur. In a system which is supplied, for example, from a cylinder of compressed gas at extremely high pressure, it is common practice to install a pressure regulator downstream of a shutoff valve on the cylinder. This arrangement minimizes the number of system components that operate at potentially hazardous high pressures. Thus, a supply of compressed gas at substantially steady, low pressure is provided to the system, despite the drop in pressure of the gas in the cylinder as the gas is released or used over time. Since the volume of a gas or fluid within a cylinder is directly related to the pressure at a given temperature, a high-pressure gage or meter is typically provided between the shutoff valve on the cylinder and the pressure regulator. Through suitable conversion analysis, an indication of supply pressure can be used to determine the volume of gas remaining in the cylinder. Thus, a typical system, utilizing a cylinder to supply pressurized gas thereto, includes a shutoff valve disposed at the cylinder outlet. Downstream of the shutoff valve is a pressure gage to provide an indication of the pressure, and likewise the volume, of gas remaining in the cylinder. The next system component is the pressure regulator that reduces the pressure of gas supplied downstream. Although the pressure regulator limits the number of components associated with the cylinder that are exposed to high pressure, this arrangement still requires the gage to be subjected to high pressure.
Heretofore, moving the gage from the preferred position in the system between the shutoff valve and pressure regulator proved highly disadvantageous since no effective substitute was provided to monitor the supply of gas remaining in the cylinder. If the gage was removed entirely from the system, no effective means remained to indicate supply fluid volume. Alternatively, positioning the gage downstream of the pressure regulator was generally considered impractical and useless since it did not convey to an operator the typical relationship of a decrease in pressure as the gas volume in the cylinder decreased.
Another problem associated with use of a meter or gage in handling certain toxic gases is gas entrapment at the dead end of a fluid line. For example, it is common practice to use Bourdon type gages that include an expansible member responsive to pressure changes. The predetermined expansion and contraction of the expansible member provides an accurate readout or indication of fluid pressure in a system. Since the construction and operation of Bourdon type gages is already well known in the art further discussion is deemed unnecessary.
When handling toxic and/or pyrophoric gases such as silane, it is necessary to purge the fluid system. For example, when initially inserted into a fluid system, air may be trapped in the expansible member of the gage. To limit the potential for adverse reaction with a pyrophoric gas, a vacuum connection is made and the air is removed from the gage. Then, the expansible member is pulse purged with nitrogen before the gage is deemed operational.